This invention relates to a method of monitoring and controlling scale formation in the flow system of a well, and more particularly to monitoring and controlling scale formation in systems where the scale is comprised of radioactive material.
The accumulation of inorganic mineral scales in oil field formations and production equipment is a major problem for the oil industry. Deposition of inorganic mineral scale in oil-bearing formations and on production tubing and equipment causes significant and costly loss of production. The primary offenders are carbonates and sulfates of calcium, barium and strontium. These compounds may precipitate as a result of changes in pressure, temperature and ionic strength of produced fluids or when connate reservoir waters mix with injected waters during secondary recovery operations. In order to avoid costly losses in production or post-scale treatments it is necessary to prevent deposition of scale downhole as well as in post production processing.
Barium and strontium sulfate scales are of particular concern because of their extremely low solubilities (10.sup.-4 to 10.sup.-5 Molar [Ba.sup.++ ] depending upon brine concentrations and temperature). At room temperature the solubility of BaS0.sub.4 in distilled water is about 2 ppm and at 80.degree. C. is about 4 ppm. In 0.5M NaCl, the solubility is 7 ppm at room temperature and about 30 ppm at 80.degree. C.; in 1.OM NaCl, about 23 and 42 ppm, respectively.
While effective measures have been developed for the removal of calcium carbonate and calcium sulfate scales, barium or strontium sulfate scaling is difficult to treat once it develops.
The most common type of scale inhibition treatment involves periodically squeezing an amount of inhibitor into the formation adjacent a producing well. This is reasonably effective in many cases, although it can lead to significant overtreatment or undertreatment if the treatment interval is not correct for the particular situation.
Another approach that has been used with some success is to monitor a flow condition, such as pressure drop through a part of the flow system, and to treat with inhibitor when the pressure drop indicates scale has formed. This approach works reasonably well for calcium scale, which can be removed with relative ease if necessary. However, for barium or strontium scales, which are difficult to remove once they have formed, it is desirable to know when scale formation has begun, and before enough accumulation has developed to provide a noticeable flow pressure drop in the system, so that an appropriate inhibitor treatment can be initiated prior to irreparable damage to the flow capacity of the system.